The operative word here is סֵפֶר (sepher) = "book", "document". There is no adjective "sacred" in the text.
The word סֵפֶר is applied to various writings such as:
- Those inside the Bible such as, Dan 9:2, Gen 5:1, Ex 24:7, Deut 28:58
- Those outside the Bible such as, Num 21:14
- Heavenly books such as Ex 32:32, 33
- a letter, 2 Sam 11:14, 15, 1 Kings 21:8
- a certificate of divorce, Deut 24:1, 3
- a deed of land purchase, Jer 32:11, Isa 50:1
... and so forth. Therefore, nothing can be deduced about the status of a document, whether canonized or not, from the meaning of the word in Dan 9:2.
I presume that "sacred books" being an interpretive translation, was selected by BSB on the basis that the "sepher" was written by the revered prophet, Jeremiah.
The extent to which various writings were "sacred scriptures" at various times is highly debated. All that can be said, is that some books were highly regarded because they had been written by recognized prophets like Jeremiah and Isaiah.
Others like the book of Jashar (Josh 10:13, 2 Sam 1:18) were not included in the Bible.